This invention relates to sintered materials, furnace materials, and protection tubes and protecting members which are to be subjected to high temperature.
Conventionally, ashes which remain after garbage is burned in an incinerator have been buried in final disposal fields. However, it has become difficult to secure a site due to strict conditions of location. Further, the detoxification of toxic pollutants such as dioxin and flon has been strictly controlled by laws and regulations. In view of the problems, in recent years, there has been an increasing demand for a furnace for melting collected incineration ash and fly ash to detoxify toxic substances. The ashes left after incineration can be reduced 1/2 to 1/4 in volume by melting them in a furnace at a high temperature. Also, toxic pollutants such as dioxin can be detoxified by pyrolysis in the furnace. Accordingly, such high temperature heat treatment is seen as promising.
In the high temperature heat treatment using a furnace, incineration ash is charged into the furnace and melted at a temperature of 1300 to 1600.degree. C. As the incineration ash is melted, some metallic elements evaporate. These metallic elements are taken out and suddenly cooled by a cooling apparatus. The thus condensed particulates are collected as metallic concentrates by a filter or the like.
On the other hand, toxic substances such as dioxin and flon are thermally broken and released as detoxified gas into the air via a gas treating apparatus. Further, the remains in the furnace are taken out as slagged granules to be effectively utilized or disposed.
For wall block of the furnace is used a refractory wall block which is made of low purity ceramics of the alumina type, magnesia type, chromia type, carbon type or composite of those types having a porosity of about 10 to 20%. The use of such a refractory furnace wall block is designed to prevent the furnace wall block from being damaged by contact with slag, and particularly to prevent a severe damage of a portion of the furnace near a slag outlet or tap opening.
Further, the furnace is provided with a heating member and a thermocouple for temperature measurement inside the furnace. However, because there are molten slag, molten salts, and vapors of incineration ashes inside the furnace, the heating member and the thermocouple are required to be protected from these substances.
A protection tube is usually used to protect the heating member and the thermocouple projecting into the interior of the furnace. Such a protection tube has an U-shaped section, and the open end of the protection tube is held between furnace wall blocks supporting the heating member. The protection tube is made of a ceramic having heat and corrosion resistance. For example, the protection tube is made of a composite ceramic of MgO--ZrSiO.sub.2 --Al.sub.2 O.sub.3, as disclosed in Japanese Unexamined Patent Publication No. SHO 51-71312.
In applying heat treatment to the incineration ash, as described above, the ash is heated and melted at a temperature of 1300 to 1600.degree. C. The protection tube and the furnace wall blocks are exposed to molten slag, which are formed as a result of melting the incineration ash, vapors or the like. Si, Al, Fe, Ca, Na and the like in the molten slag gradually penetrate and corrode into the ceramics forming the protection tube and the furnace materials, consequently changing the properties of the ceramics gradually. This leads to a deteriorated strength, cracks and damages, and partial melting of the protection tube and furnace wall blocks. Accordingly, it has been very difficult to use the protection tube and the furnace wall blocks over a long period of time. This phenomenon has been particularly conspicuous in the outer surface of the bottom of the protection tube.
Further, a part of the vapor components enters through a narrow clearance between the protection tube and the furnace wall blocks holding the protection tube, causing the furnace wall block to change in its properties, and corroding and melting them. A glass component of the molten furnace material deposits on a bottom portion of the protection tube after running down along the heating member, and considerably corrodes the inside of the protection tube. The molten glass corrodes the protection tube at a corrosion rate several times as fast as the corrosion from the outer surface of the protection tube. In a worst case, such corrosion causes a through hole in the bottom wall of the protection tube, considerably shortens the life of the heating member, and gives a fatal damage to the life of the furnace. The problem of molten glass has been crucial.